In a statement to parliament, the Foreign Secretary said that there was increasing evidence that chemical weapons – in particular the nerve gas sarin – had been used by forces loyal to President Assad.

There is a growing body of limited but persuasive information showing that the regime used – and continues to use – chemical weapons," he said. "We have physiological samples from inside Syria which have shown the use of sarin, although it does not indicate the scale of that use. Our assessment is that chemical weapons use in Syria is very likely to have been by the regime."

He added that there was no evidence of use of such weapons by the rebel forces.

Earlier this month Carla Del Ponte, the UN human rights investigator, said that there were "strong, concrete suspicions" that the rebels had used sarin. White House spokesman Jay Carney later insisted Ms Del Ponte was wrong, and that the Assad regime was probably guilty of using such weapons, although the US had still not reached a definitive conclusion on the matter.

Mr Hague told the House that there were "no options off the table" if Mr Assad failed to attend peace negotiations. The US and Russia have agreed to hold a conference on the Syrian conflict in Geneva, likely to be in early June.

"We must make clear that if the regime does not negotiate seriously at the Geneva conference, no option is off the table," he said. "There remains a serious risk that the Assad regime will not negotiate seriously."

The Syrian leader has so far remained uncommitted to the talks, saying that he supported them in principle but that he was sceptical about the motives of the organisers and that "believing that a political conference will stop terrorism on the ground is unreal."

However, Mr Hague said that the coming weeks would see the "most intense diplomacy yet to bring together members of the United Nations Security Council" to agree on a course of action.

The Foreign Secretary will travel to Jordan on Tuesday ahead of a meeting with John Kerry on Wednesday. On Monday he will attend a meeting of EU foreign ministers in Brussels, where once againSyriawill be top of the agenda.

"With every week that passes we are coming closer to the collapse of Syria and a regional catastrophe, with the lives of tens of thousands more Syrians at stake. We are determined to make every effort to end the carnage," he said.

Mr Hague said that half a million refugees had fled the country in the past 10 weeks alone, and that the UN estimates that 3.5 million Syrians - of 15 per cent of the population - will have abandoned their homes by the end of the year.

He said that Britain had so far contributed £171.1m to help the Syrian people, including £30m announced last week by the Prime Minister in support for people in need in areas held by the opposition or contested. He said that the UK had provided food for 150,000 people, given equipment to the Jordanian armed forces to help provide for the refugees, and assisted with the support of Lebanese forces – providing for four border towers to protect their frontier.

"The international focus must above all be on ending the crisis," he said, adding that Britain had not armed any side during the Arab Spring conflicts, but "the case for further amendments to the arms embargo is compelling."

He said: "No decision has been made to go down this route, and if we were to pursue this, it would be under the following conditions: in coordination with other nations, in carefully controlled circumstances, and in accordance with our obligations under national and international law.

"The United Kingdom and France are both strongly of the view that changes to the embargo are not separate from the diplomatic work, but essential to it."

He added: "We have to be open to every way of strengthening moderates and saving lives."